1. The Field of the Invention
The present invention is related to methods for preparing feed stock of gas generant formulations, more particularly, the present invention is related to methods for preparing non-azide gas generant compositions into a form which may be pressed, extruded or otherwise processed into a final form.
2. Technical Background
Gas generating chemical compositions are useful in a number of different contexts. One important use for such compositions is in the operation of inflatable automotive safety restraint systems, commonly referred to as "air bags." Air bags are gaining in acceptance to the point that many, if not most, new automobiles are equipped with such devices. Indeed, many new automobiles are equipped with multiple air bags to protect the driver and passengers.
In the context of automobile air bags, sufficient gas must be generated to inflate the device within a fraction of a second. Between the time the car is impacted in an accident, and the time the driver would othrwise be thrut against the steering wheel or dash, the air bag must fully inflate. As a consequence, nearly instantaneous gas generation is required.
There are a number of additional important design criteria that must be satisfied. Automobile manufacturers and others set forth the required criteria which must be met in detailed specifications. Preparing gas generating compositions that meet these important design criteria is an extremely difficult task. These specifications require that the gas generating composition produce gas at a required rate. The specifications also place strict limits on the generation of toxic or harmful gases or solids. Examples of restricted gases include carbon monoxide, carbon dioxide, NO.sub.x, SO.sub.x, and hydrogen sulfide.
Automobile manufacturers have also specified that the gas be generated at a sufficiently and reasonably low temperature so that the occupants of the car are not burned upon impacting an inflated air bag. If the gas produced is overly hot, there is a possibility that the occupant of the motor vehicle may be burned upon impacting a just deployed air bag. Accordingly, it is necessary that the combination of the gas generant and the construction of the air bag isolates automobile occupants from excessive heat. All of this is required while the gas generant maintains an adequate burn rate. In the industry, burn rates in excess of 0.5 inch per second (ips) at 1,000 psi, and preferably in the range of from about 1.0 ips to about 1.2 ips at 1,000 psi are generally desired.
Another related but important design criteria is that the gas generant composition produces a limited quantity of particulate materials. Particulate materials can interfere with the operation of the supplemental restraint system, present an inhalation hazard, irritate the skin and eyes, or constitute a hazardous solid waste that must be dealt with after the operation of the safety device. The latter is one of the undesirable, but tolerated in the absence of an acceptable alternative, aspects of the present sodium azide materials.
In addition to producing limited, if any, quantities of particulates, it is desired that at least the bulk of any such particulates be easily filterable. For instance, it is desirable that the composition produce a filterable, solid slag. If the solid reaction products form a stable material, the solids can be filtered and prevented from escaping into the surrounding environment. This also limits interference with the gas generating apparatus and the spreading of potentially harmful dust in the vicinity of the spent air bag which can cause lung, mucous membrane and eye irritation to vehicle occupants and rescuers.
Both organic and inorganic materials have also been proposed as possible gas generants. Such gas generant compositions include oxidizers and fuels which react at sufficiently high rates to produce large quantities of gas in a fraction of a second.
At present, sodium azide is the most widely used and accepted gas generating material. Sodium azide nominally meets industry specifications and guidelines. Nevertheless, sodium azide presents a number of persistent problems. Sodium azide is relatively toxic as a starting material, since its toxicity level as measured by oral rat LD.sub.50 is in the range of 45 mg/kg. Workers who regularly handle sodium azide have experienced various health problems such as severe headaches, shortness of breath, convulsions, and other symptoms.
In addition, sodium azide combustion products can also be toxic since molybdenum disulfide and sulfur are the oxidizers which are commonly used with sodium azide. The reaction of these materials produces toxic hydrogen sulfide gas, corrosive sodium oxide, sodium sulfide, and sodium hydroxide powder. Rescue workers and automobile occupants have complained about both the hydrogen sulfide gas and the corrosive powder produced by the operation of sodium azide-based gas generants.
Increasing problems are also anticipated in relation to disposal of unused air bag systems in demolished cars. The sodium azide remaining in such supplemental restraint systems can leach out of the demolished car to become a water pollutant or toxic waste. Indeed, some have expressed concern that sodium azide, when contacted with battery acids following disposal, forms explosive heavy metal azides or hydrazoic acid.
In response to the disadvantages attendant with the use of azide-based fuels for gas generant compositions, many new compositions have recently been developed. These compositions include those disclosed and claimed in co-pending U.S. patent application Ser. No. 08/101,396, filed Aug. 2, 1993 and entitled Bitetrazoleamine Gas Generant Compositions and Methods of Use, and incorporated herein by reference. As disclosed in that patent application, a bitetrazoleamine ("BTA"), or a salt or a complex thereof is used as a fuel with copper oxide employed as a preferred oxidizer. The composition can comprise from about 15 to about 35 weight percent fuel and from about 60 to about 85 weight percent oxidizer.
Other recently developed gas generant compositions include those disclosed and claimed in co-pending U.S. patent application Ser. No. 08/103,768, filed Aug. 10, 1993 and entitled Thermite Compositions for use as Gas Generants, and incorporated herein by reference.
While many of these new compositions may be easily processed on a laboratory scale, such processing techniques do not readily lend themselves to full-scale processing. Bulk processing of gas generant materials is generally conducted in two phases: a "feed stock" is initially prepared from which a final gas generant product may be produced. The final product is generally produced through pressing or extrusion. Currently, the most prevalent final product is a pressed tablet having a diameter of approximately 0.25 inches. In an automobile gas bag, many of these tablets, sometimes referred to as "pellets" or "pills," may be utilized to obtain a sufficient quantity of gas generant to inflate the gas bag.
Pellets are generally produced by placing a quantity of feed stock in a pellet press. The feed stock is then physically pressed into a die having the shape of that desired of the pellet. After pressing, the pellet may be removed from the mold.
In order to effectively produce gas generant pellets, the feed stock must have bulk flow characteristics which enable it to easily flow from a pellet press feed bin into the die. Also, the feed stock must effectively release from the die when pressing is complete. The pellet which is produced must also have substantial crush strength so that it will not crumble or otherwise erode when subjected to the forces to be encountered when positioned in the steering wheel of a vehicle over a period of several years.
A non-azide based gas generant formulation which is substantially superior to an azide-based formulation in terms of ballistics and safety cannot be effectively utilized unless it can be processed efficiently. One difficulty faced by some new formulations is that the composition must hold true on a macro-molecular level. In contrast, typical azide-based compositions can tolerate small agglomerations of virgin sodium azide unaccompanied by an oxidizer. Because the composition tolerances are quite lenient, it is not necessary to process such azide-based compositions to ensure that each granule of sodium azide is physically attached to an oxidizer.
Many of the new, superior performing, generant compositions, however, appear to require strict composition balances at the macro-molecular level. Hence, many of the known processing techniques for azide-based compositions will not work for processing new, non-azide based compositions without significant process alterations.
From the foregoing, it will be appreciated that it would be an advancement in the art to provide methods for processing gas generant compositions to produce feed stock which can economically and efficiently be pressed to form a gas generant tablet which will readily release form the die and have sufficient crush strength to meet industry standards.
Indeed, it would be an additional advancement in the art to provide such methods for processing gas generant compositions, particularly new, non-azide based compositions, which methods will permit bulk processing to achieve more precise compositions on a macro-molecular level.
Such a methods are disclosed and claimed herein.